Project Summary B cell terminal differentiation gives rise to antibody-secreting cells (ASCs) that provide immunity against pathogens. Vaccines induce B cell terminal differentiation to provide targeted immunity. When B cell terminal differentiation is dysregulated, diseases such as autoimmunity and cancer result. To improve vaccine design and treatments for B cell-based diseases, we must understand the molecular mechanisms that regulate B cell terminal differentiation. Recent evidence suggests that epigenetic reprogramming guided by key transcription factors drives B cell terminal differentiation. Of note, the histone demethylase LSD1 has been shown to directly interact with BLIMP1 to mediate suppression of the B cell transcriptional program in ASCs. LSD1 demethylates histone 3 lysine 4 mono- and di-methylation (H3K4me1/2) in order to decommission active enhancers and promoters. In depth analysis of the impact that specific epigenetic modifying proteins have on B cell terminal differentiation as well as the chromatin modulations they impose will shed light on the molecular mechanisms that regulate this process, thus we propose to further study the role of LSD1 in this process. Preliminary experiments show a 65% and 58% reduction in ASCs generated from ex vivo and in vivo differentiation of LSD1-deficient B cells, respectively, indicating that LSD1 is highly important for B cell terminal differentiation. Also, RNA-seq and ATAC-seq show that without LSD1, both naive B cells and ASCs exhibit targeted gene upregulation and gain of chromatin accessibility, suggesting that LSD1 mediates gene silencing through mechanisms that promote the closing of chromatin. These data lead us to hypothesize that LSD1 facilitates proper B cell terminal differentiation by repressing B cell program genes in ASCs via demethylation of H3K4me1/2 at promoter and enhancer regions. In aim 1, we will further define the functional role of LSD1 throughout B cell terminal differentiation. LSD1 and B cell terminal differentiation are conserved between humans and mice, so we will utilize a mouse model in which Lsd1 is conditionally deleted in B cells. To induce B cell terminal differentiation, we will immunize mice with T cell-dependent and -independent model antigens. We will use these systems in conjunction with flow cytometry-based assays, serological assays, and immunohistochemistry to assess the contribution of LSD1 to the B cell terminal differentiation stages of activation, proliferation, the germinal center reaction, and the transition of activated B cells into ASCs. In aim 2, we will determine the molecular mechanism by which LSD1 regulates B cell terminal differentiation by performing ChIP-seq on LSD1-sufficient and LSD1-deficient B cell populations prior to and following immunization. Preliminary sequencing data and ChIP-seq data analyses will be used to guide the development of genetic and molecular assays to probe gene relevance to LSD1-based regulation of B cell terminal differentiation. This work will improve our knowledge of how human B cell terminal differentiation is regulated and can be used to further improve the development of vaccines and treatments for B cell-based diseases.